In a copending application for Letters Patent of the United States filed Aug. 6, 1979, by one of the joint inventors herein and having Ser. No. 063,797, there is disclosed a falling film heat exchanger fabricated from non-metallic heat exchange surfaces having vertically disposed mutually parallel single sheet curtains functioning as heat transfer surfaces and down which a hot liquid gravitates in the form of a film while the heat is being taken therefrom and transferred to a second relatively cooler liquid or gas passing in heat exchange relation thereto on the opposite side of each such curtain. The present invention relates to an improvement of the aforementioned heat exchanger which, among other things, replaces the single-sheet film-carrying curtain with a closely spaced pair of sheets cooperating to define a vertically oriented flow-through pocket in which a curtain of relatively warm liquid gravitates down between the pair of sheets so as to define a fluid coupling therebetween, primarily due to capillary forces.
One disadvantage of the single sheet heat exchange surface, as disclosed in the aforesaid patent application, is the instability of each of the sheets under the influence of air moving over the surface of the sheet. The reduction of pressure caused by the moving air allows the flexible sheet to deform from a flat surface, distorting the air passage and blocking the air flow and sometimes breaking the continuity of the liquid film on the sheet. This problem has been alleviated some by the provision of pressure relief ports and other devices designed to equalize the pressure across the heat exchange surfaces. However, these solutions have not been totally satisfactory, resulting in operating inefficiencies and package density limitations and have led to the discoveries embodied in this invention where the capillary bonded pocket provides self-pressure regulation under the influence of moving air and neither the falling liquid film on the outside of the pocket nor the dynamic air space dimensions is adversely affected.
Confinement of a descending liquid between two wettable sheets in close proximity provides a liquid curtain which is dynamically stable over a very wide range of liquid flow rates. The heat exchanger formed by passing a second liquid over the outside surfaces of the pockets or a gas in close proximity to a plurality of the liquid curtains within the pockets can typically operate over a 20 to 1 ratio of liquid flow rates. This turndown ratio of 20 to 1 compares to a typical turndown capability of 2 to 1 for a normal exposed surface falling liquid film. The heat exchanger can also operate at much lower unit flow rates than a typical exposed surface falling film type. These capabilities are due to the contribution to stability by surface tension forces between the liquid and the two pocket forming sheets in close proximity. The high turndown ratio and the low flow capability make possible a wide range of applications heretofor impractical with a falling film heat exchanger.
The fluid flow characteristics of the liquid curtain within each pocket are such that liquid residence time in the pocket increases and the curtain surface area per total flow increases as the unit flow on a segment of the curtain is decreased (same property as any type of falling film). This, combined with low flow capability, allows heat exchange performance to be comparable to ultra-high surface devices such as conventional cooling towers and modern extended surface heat exchangers. Such performance is far beyond the capability of other falling film heat exchangers.
The physical nature of the water curtain mandates that the thickness of the curtain is exactly defined by the perimeter flow rate and because the confining sheets cannot resist loads in the transverse direction, they conform to the spacing determined by curtain flow theory. Curtain flow theory shows that the curtain thickness will range between 0.01 and 0.03 inches for 95% of all unit flow rates representing practical applications of the heat exchanger. The weight of this very thin curtain can easily be supported by the two sheets and the physical space required by the curtain (cubic feet fluid cavity per square foot of surface) is by far the lowest of any prior art falling film device.
Fluid stability of the water curtain requires that air not be pulled into the space between the wettable sheets at the bottom of the pocket and all that is necessary to satisfy this requirement is that the static pressure at the pocket outlet be infinitesimally less than the static pressure on the surface of the curtain. Under these circumstances the liquid curtain will purge gasses from the pocket and achieve and maintain stability due to the natural forces within the system. Two novel approaches for accomplishing this result are carried out in this invention.
The first approach provides a sealed air space which includes the air space into which the fluid exits from the bottom collection manifolds (i.e., the manifolds collecting the liquid flowing through the pockets) and the downstream plenum into which the air flows from the between-pocket air ducts. This guarantees that the total pressure at the fluid outlet is lower than the total pressure anywhere on the sheet surfaces. (Total pressure is defined as static pressure plus velocity pressure.) The static pressure difference will then be negative or only slightly positive. In most cases the effect of a slightly positive static pressure will be overcome by capillary forces holding the sheets to the water curtain at the curtain flow exit.
The second approach is to exit the fluid into a closed duct or manifold such that the fluid totally fills the duct which is configured in such a manner that the static pressure at the bottom of the pocket is lower than the static pressure on its surfaces. This can be accomplished by exiting the fluid from the duct at a slightly lower elevation than at the bottom of the pocket, allowing the weight of the free-falling water column to produce a negative pressure.
So far as Applicants are presently aware, no prior art teaching exists of a double-sheeted wettable pocket wherein hot liquid flows down between the sheets, holding them apart, and at the same time defining a fluid coupling bridging the gap thus formed. U.S. Pat. No. 3,913,667 details a double-walled flexible curtain made of flexible sheets; however, the space therebetween is such that no liquid film bridge or bond is present and the only fluid connection exists at the bottom where both sheets are immersed in a pool of collected fluid. Moreover, instead of the pressure on the inside and outside of the double-sheeted enclosure being everywhere in equilibrium, as taught herein, the interior pressure is greater than the exterior so as to force the sheets apart and into contact with a restraining grid.
It is, therefore, the principal object of the present invention to provide a heat exchange process which advantageously utilizes a uniquely defined and novel curtain of falling liquid which, because of its peculiar physical properties, is dynamically stable in an environment of moving gas, irrespective of flow rate, temperature, viscosity or fluid density.
A second object of the present invention is to provide a modified falling film heat exchanger characterized by a plurality of suspended flow-through pockets, the confining sheets of which are forced apart and at the same time bonded together by a film of liquid introduced therebetween to maintain an equilibrium condition inside and outside thereof at all points on its unrestrained surface.
An additional object of the within described invention is to provide a modified falling film heat exchanger utilizing flexible membranes for heat exchange surfaces that require no special provision for balancing the pressures thereacross or for special accommodation of diffferent flow rates of coolant.
Another object of the invention is to provide a modified falling film heat exchanger having a possible turndown ratio of 20 to 1 and a capability to operate at low flow rates without adversely affecting the character and stability of the liquid curtain.
Another object of the invention is to provide a heat exchanger that can alternately function as an evaporative cooler or an indirect air-liquid sensible heat exchanger, depending on whether a secondary cooling liquid is flowing.
Still another object is to provide a unique modular system for suspending the pocket forming sheets in mutually-spaced parallel relation inside a framework allowing for the flow of air therebetween as well as the gravity flow of liquid through the interior and down both exterior surfaces of the pockets.
Further objects are to provide a modified falling film heat exchanger which is simple, relatively maintenance-free, easy to service, less expensive than its metal counterparts, efficient, and substantially compact.